The principle of a cycle of coffee beverage preparation is hereafter explained, by way of example, for a fully automatic coffee machine.
The quantity of coffee powder necessary for the preparation of the beverage is introduced into a brewing chamber. Then, this coffee powder is compressed to a predetermined volume by means of a movable piston, and water is taken from a fresh water container and pressurized by a pump. Then, this water is passed through a boiler and thereafter is allowed to brew the compressed coffee powder contained in said brewing chamber. Finally, the freshly prepared coffee beverage flows out from the brewing chamber and, through a suitable outlet, into a suitably placed coffee cup.
One disadvantage inherent to all known fully automatic coffee machines is that residual water remains in the coffee machine after each cycle of beverage preparation. Said residual water is the water which was drained by the water pump from the fresh water container and had gathered after said cycle of beverage preparation inside the coffee machine, generally at the lowermost points of the water conducting pipes and eventually in the brewing chamber, and which cannot flow off. The quantity of the residual water which remains in the coffee machine largely depends on the construction of the machine.
Said residual water remains in the machine until a new cycle of beverage preparation is started. Water which had gathered upstream of the boiler again passes through said boiler in the next cycle of beverage preparation. Thus, there are no objections with respect to the microbiological quality of this water. However, residual water which had gathered between the boiler and the beverage outlet is not again heated in a new cycle of beverage preparation, but mixes with the brewing water flowing through. Since the residual water eventually remains in the coffee machine for a prolonged time, there is the danger that said stale residual water is microbiologically critical.
The problems of said residual water gathered in the coffee machine exists with most of the coffee machine used today, and in particular with fully automatic coffee machines in which the brewing water flows upwardly through the coffee powder in the brewing chamber, as well as with fully automatic coffee machines which comprise a hydraulically actuated piston for compressing the coffee powder.
Apart from the objections with respect to the hygiene, said stale residual water causes further disadvantages. One of them is that the taste of the freshly brewed coffee beverage is negatively influenced by the fact that the stale residual water is mixed with the fresh water during brewing. Furthermore, said stale water, which is cold, when mixed with the just heated fresh water, reduces the temperature of the latter. If an espresso is prepared, this in particular results in a low temperature of the beverage.
With semi-automatic coffee machines, in which the coffee powder is to be dosed by the user onto a filter carrier, the problems of the residual water remaining in the coffee machine is generally solved by recommending to the user to preheat the coffee cup provided for receiving the fresh coffee beverage, before the actual cycle of beverage preparation, by starting the coffee machine without having dosed coffee powder onto the filter carrier, thus making to run only hot water through the coffee machine into the coffee cup. Obviously, in this way the remaining residual water can be removed from the coffee machine. However, this method cannot be applied to fully automatic coffee machines, since the dosing of the coffee powder is effected fully automatically, thus excluding the possibility of running only hot water into the coffee cup.